It's a decent game but it's in no way comparable to Planescape Torment. The story is a letdown precisely because it tries too hard to ape its source material: the villain is someone who used to be a part of you (or, in this case, someone who used you as a vessel) and the consequences of his past actions are a main theme of the game, just like in PST. Except here it feels way less personal and urgent than in PST. In PST you can die as often as you want but always wake up again, and sometimes you lose your memory and your personality changes. That's a bad thing. You want to find out what's happening to you and how you can stop this from happening.
In Numenera you're a cast-off vessel of some godlike dude. And... now you can pretty much do what you want. You could retire in a nice coastal manor if you wanted and there wouldn't be any major personal consequences for you. Your stakes aren't half as personal as they were in PST.
Essentially the game takes PST's themes and motives but fails to create the same sense of personal connection to the plot.
Also, the writing is ridiculously overwritten. Yes, PST was a very wordy game, but its writing had a good flow to it. It was elegant and often witty. Numenera's writing often reads like it's been sprinkled with fancy adjectives just to look high-brow and complex. And on top of that, a lot of the side quest content is just weird for the sake of weird without tying into any overarching theme. PST was weird too, but most of the sidequests and factions had some relation to the story's or the overall setting's themes. In Planescape, the "rule of three" is a thing and you encounter it in action several times. There's the idea that if enough people believe in something, it becomes real, and that actually happens several times. Planescape is weird but it's consistent in its weirdness.
Meanwhile in Numenera, anything and everything can happen, and there doesn't need to be any real reason for it. There can be cyberpunk elements, biopunk elements, items that work like fantasy magic, items that work like Star Trek technology, etc etc. It doesn't care about consistency, it just throws a lot of weird shit at you. Which isn't a bad approach by itself and would be excellent in an open world game, but it's not an open world game. It's a story focused RPG. And nothing outside of the story really ties into the story or its themes properly, so it just ends up feeling disjointed and like it doesn't know what it wants.
It's not a bad game, but it makes plenty of mistakes and is mediocre as a consequence.